1. Field of the Invention
The present invention relates to a buttonhole sewing machine having a thread cutting and clamping device.
2. Background Art
A known stitch-group sewing machine, in particular a button-hole sewing machine, has a thread cutting and clamping device (Federal Republic of Germany Pat. No. 2,053,559) which at the end of a working cycle cuts the needle thread and holds the needle-side end of the thread fast so as to prevent its becoming unthreaded when the sewing recommences. The disclosures of this, and all other prior art materials cited herein, are expressly incorporated by reference.
This known button-hole sewing machine also has, between the sewing needle and the thread cutting and clamping device, a lowerable hold-down device which, in the immediate vicinity of the sewing needle, presses with slight force against the part being sewn. In this way, particularly in the case of large-mesh knitted articles, the high-pile material being sewn is held down in the immediate region of the place of sewing, so that a suitable sewing result is obtained.
In addition to cutting the needle thread, the thread cutting and clamping device also brings the end of the needle thread into a position in front of the sewing needle and holds it fast, in such a way that it is sewn over or covered by the zigzag seam produced.
During the sewing of the first stitches for the first button-hole caterpillar pattern, the thread cutting and clamping device in the known button-hole sewing machine does not carry out any movement at the place intended for the commencement of the sewing. At this time, the material-presser basket, in known cooperation with a lengthwise pusher, transports the sewing part clamped between the two of them in the direction toward the sewing needle. This produces a thread-tensioning force, which increases with the number of zigzag stitches, in the end of the needle thread which is held clamped. In this way, the hold-down device is raised, the end of the needle thread which is held fast extending obliquely upwards, starting from the place of sewing, towards the thread cutting and clamping device. As a result of this, the zigzag stitches of the first button-hole caterpillar pattern which are now formed are pulled up when the end of the needle thread which is held fast is oversewn, and this with an increasing amount of force. At a given point, the thread tensile force has reached such a value that the end of the needle thread which had previously been held clamped is pulled out of the thread clamping device. Since now the thread tensioning force in the free end of the needle thread has dropped to the value of zero, a normal button-hole caterpillar pattern with flat-formed zigzag stitches is produced from this time on.
This known button-hole sewing machine, therefore, has the disadvantage that in the initial region of the first button-hole caterpillar pattern, some of the zigzag stitches are pulled upward, but are then, however, pulled properly later in the sewing procedure. This defect is objected to by those skilled in the art who wish to produce quality button holes.